1. Field of the Invention
The present invention relates to a semiconductor light emitting device, a method for manufacturing the same, and a method for forming an underlying layer.
2. Description of the Related Art
As a semiconductor laser having low threshold current Ith, a semiconductor laser having a separated double hetero junction (SDH) structure that can be formed through one time of an epitaxial growth step (hereinafter, referred to as an SDH semiconductor laser) is known from e.g. Japanese Patent No. 2990837.
For this SDH semiconductor laser, initially a projection part extending along the {110}A plane direction is formed on a substrate having the {100} plane as its major surface. Subsequently, through crystal growth over the major surface of this substrate, a light emitting part arising from stacking of compound semiconductor layers is formed on the {100} plane of the projection part (for convenience, referred to as the projection surface). The light emitting part has e.g. a structure arising from sequential stacking of a first compound semiconductor layer of a first conductivity type, an active layer, and a second compound semiconductor layer of a second conductivity type. The sectional shape obtained when this light emitting part is cut along a virtual plane perpendicular to the extension direction of the projection part is e.g. a triangle, and the side surface (oblique surface) of the light emitting part is the {111}B plane. In general, the {111}B plane is known as a non-growth surface in MOCVD (Metal Organic Chemical Vapor Deposition referred to also as MOVPE (Metal Organic Vapor Phase Epitaxy)), except for special crystal growth conditions. Therefore, in the case of the SDH semiconductor laser, after the light emitting part whose side surface is the {111}B plane is formed, “self growth stop” of the crystal growth of the light emitting part is kept even if the MOCVD is continued thereafter. The inclination angle (α) of the {111}B plane is 54.7 degrees.
In the present specification, the crystal planes shown below are represented as the (hkl) plane and the (hk-l) plane, respectively, for convenience.
(hkl) plane
(hk l)plane
In addition, the directions shown below are represented as the [hkl] direction and the [hk-l] direction, respectively, for convenience.
[hkl]direction
[hk l] direction
On the other hand, the {100} plane part as the major surface of the substrate except the projection part (for convenience, referred to as the recess surface) does not involve a non-growth surface. Thus, if the MOCVD is continued, a compound semiconductor layer formed through crystal growth from the recess surface will completely cover the light emitting part in the self growth stop state in time. The compound semiconductor layer formed through crystal growth from the recess surface has, on the second compound semiconductor layer, a structure arising from sequential formation of a layer for adjustment of the current block layer position (hereinafter, referred to simply as the adjustment layer) a current block layer, and a burying layer. In general, controlling the thickness of the adjustment layer makes it possible to form a structure that permits current injection only to the active layer of the light emitting part through formation of the current block layer at an intermediate phase before the compound semiconductor layer formed through crystal growth from the recess surface covers the light emitting part (in particular, when the upper surface of the compound semiconductor layer is about to reach both the side surfaces of the active layer formed in the light emitting part).
In this manner, for the SDH semiconductor laser, the respective compound semiconductor layers can be formed based on one time of a crystal growth step. In addition, the active layer can be completely surrounded by compound semiconductor layers favorable for light confinement by selecting materials whose energy band gaps are sufficiently wider than that of the active layer, i.e., materials having lower refractive indexes, as the materials used for the compound semiconductor layers that vertically sandwich the active layer in the light emitting part (the first compound semiconductor layer and the second compound semiconductor layer) and the materials used for the current block layer, the burying layer, and the adjustment layer located outside the light emitting part. Due to this feature, the shape of a beam emitted from the semiconductor laser whose light emitting surface is the end surface of the projection part can be brought close to a perfect circle. That is, as the far field pattern (FFP), the following relationship can be achieved.θ//≈θ⊥
Furthermore, depending on e.g. the efficiency of coupling with a lens, it is often needed that the shape of a beam emitted from the semiconductor laser is an ellipse. For such a case, the θ// of the FFP can be set small e.g. by employing a so-called flare-stripe structure, in which the width of the projection part near the ends of the projection part is increased (refer to e.g. Japanese Patent No. 3399018). Moreover, employing the flare-stripe structure can achieve high light output.